NOx, Hg, and SO2 removal using ammonia

Abstract

A process and apparatus for removing SO2, NO, and NO2 from a gas stream having the steps of oxidizing a portion of the NO in the flue gas stream to NO2, scrubbing the SO2, NO, and NO2 with an ammonia scrubbing solution, and removing any ammonia aerosols generated by the scrubbing in a wet electrostatic precipitator. The process can also remove Hg by oxidizing it to HgO and removing it in the wet electrostatic precipitator. Ammonium sulfate, a valuable fertilizer, can be withdrawn from the scrubbing solution.

Description

BACKGROUND OF INVENTION[0001]a. Field of the Invention[0002]This invention relates to methods and apparatuses for removing NOx and SO2 from a gas stream.[0003]b. Description of the Related Art[0004]Fossil fuels are burned in many industrial processes. Electric power producers, for example, burn large quantities of coal, oil, and natural gas. Sulfur dioxide (“SO2”), nitrogen oxide (“NO”), and nitrogen dioxide (“NO2”) are some of the unwanted byproducts of burning any type of fossil fuel. Mercury (“Hg”) is often also found in fossil fuels. These byproducts are known to have serious negative health effects on people, animals, and plants, and a great deal of research has been done to find a way to economically remove them from flue gas streams before they enter the atmosphere.[0005]SO2 is often removed from gas streams (“desulfurization”) by scrubbing the gas with an aqueous ammonium sulfate solution containing ammonia. Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807,...

AI Technical Summary

Benefits of technology

[0024]Maintaining a relatively high pH has several benefits. It increases the speed of absorption of SO2. It increases the ratio of sulfite available in solution compared to bisulfite, which facilitates the oxidation of SO2 and reduction of NO2. The ratio of sulfite to bisulfite is highly dependent on pH level. From these benefits, it follows that the absorption vessel, shown as item 44 in FIG. 2, can be substantially smaller than that used to scrub the same amount of SO2 in a conventional limestone scrubber which is the most typical SO2 scrubber in use today. In addition, the amount of scrubbing liquid required and the liquid to gas ratio can be reduced. It is estimated that the size of the absorption vessel 44 can be reduced by half, and the liquid to gas ratio can be reduced by a third. Because the cost of the absorption vessel and liquid circulating equipment represent a large fraction of the total cost of a scrubber, the ability to substantially reduce the size of the vessel and associated pumps and piping is a major advantage of the present invention over the prior art.

[0025]Although FIG. 1 shows ammonia being added at this step, ammonia in the form of ammonium hydroxide can be added instead. The ammonia reacts with the gas stream output from the oxidizing step, forming ammonium sulfite and ammonium bisulfite. The likely chemical reactions in this step are as follows:NH3+H2O+SO2#NH4HSO3  (6)NH4HSO3+NH3#(NH4)2SO3  (7)2NH4OH+SO2#(NH4)2SO3+H2O  (8)

[0026]An oxidation inhibitor can be added at this step to inhibit the oxidation of sulfite to sulfate before the sulfite can perform its NO2 reduction function. Examples of oxidation inhibitors include thiosulfate and thiourea.

[0027]The ammonium bisulfite and ammonium sulfite reacts with the NO and NO2 to form ammonium sulfate. Ammonium sulfate is well known as a valuable agricultural fertilizer. The likely reactions that take place in this step are as follows:2NO2+4(NH4)2SO3#4(NH4)2SO4+N2  (9)NO+NO2+3(NH4)2SO3#3(NH4)2SO4+N2  (10)

[0028]Most of the HNO3 that may have been formed by further oxidation of NO and NO2, and / or created by a DBD reactor, will react with ammonia and form ammonium nitrate, also known to be a valuable agricultural fertilizer, according to the following formula:HNO3+NH3#NH4NO3  (11)

[0029]In a similar way, most of the sulfuric acid created by the DBD reactor will react with the solution and form ammonium bisulfate, which will then be oxidized to ammonium sulfate. As one can see from the above equations, the process removes SO2, NO, and NO2 from the gas stream, and produces ammonium nitrate, ammonium sulfate, and nitrogen. Over time, the ammonium sulfate and ammonium nitrate will concentrate in the aqueous ammonia solution and precipitate out of solution. The solid precipitate can then be removed from the scrubber and processed for use as fertilizer.

Problems solved by technology

These byproducts are known to have serious negative health effects on people, animals, and plants, and a great deal of research has been done to find a way to economically remove them from flue gas streams before they enter the atmosphere.

This process, however, does not remove SO2, and requires the addition of chlorine or ozone into the system by some other means.

However, both U.S. Pat. Nos. 4,029,739 and 4,035,470 require the addition of chlorine to a gas stream that is eventually released to the atmosphere, creating a serious safety concern.

This is also called “ammonia slip.” Free ammonia in the atmosphere would be a serious health and environmental hazard.

Firnhaber dismisses the possibility of aerosol removal means due to prohibitive investment costs and high pressure loss, for instance.

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